The present invention relates to a process for the preparation of thermoplastic polyurethanes by reacting
a) organic and/or modified organic polyisocyanates with PA1 b) at least one oligomeric polyhydroxy and/or polyamino compound having an average molecular weight of from 400 to 10 000 g/mol, PA1 c) chain extenders having at least two Zerewitinoff-active hydrogen atoms and an average molecular weight of less than 400 g/mol, PA1 d) catalysts, PA1 e) compounds which are less than difunctional with respect to isocyanates and/or isocyanates which are less than difunctional and PA1 f) assistants and/or additives PA1 a) organic and/or modified organic polyisocyanates with PA1 b) at least one oligomeric polyhydroxy and/or polyamino compound having an average molecular weight of from 400 to 10 000 g/mol, PA1 c) chain extenders having at least two Zerewitinoff-active hydrogen atoms and an average molecular weight of less than 400 g/mol, PA1 d) catalysts, PA1 e) compounds which are less than difunctional with respect to isocyanates and/or isocyanates which are less than difunctional and PA1 f) assistants and/or additives PA1 n--is the speed of the extruder screws in [min.sup.-1 ], PA1 D.sub.G --is the smallest internal diameter of the extruder barrel in [cm] and PA1 V--is the free reactor volume available between the extruder barrel and the screw elements and any other elements as equipment on the extruder shafts in [cm.sup.3 ], PA1 a) Examples of suitable organic and/or modified organic polyisocyanates (a) are aliphatic, cycloaliphatic, araliphatic, aromatic and heterocyclic diisocyanates. Specific examples are aliphatic diisocyanates, such as hexamethylene diisocyanate, preferably cycloaliphatic diisocyanates, such as isophorone diisocyanate, cyclohexane 1,4-diisocyanate, 1-methylcyclohexane 2,4- and 2,6-diisocyanate and the corresponding isomer mixtures, and dicyclohexylmethane 4,4'-, 2,4'- and 2,2'-diisocyanate and the corresponding isomer mixtures. PA1 b) Preferred relatively high molecular weight oligomeric polyhydroxy and/or polyamino compounds (b) having average molecular weights of from 400 to 10 000 g/mol are polyester diols, polystercarbonate diols and polyether diols, for example polyester diols obtained from straight-chain or branched aliphatic and/or cycloaliphatic diols and aliphatic dicarboxylic acids, in particular adipic acid. However, they may also contain minor amounts of aromatic dicarboxylic acids, in particular phthalic acid and, if required, also terephthalic acid, and the hydrogenation products thereof. Hydroxypolycarbonates and hydroxypolycaprolactones are also suitable. PA1 c) Suitable chain extenders (c) having at least two Zerewitinoff-active hydrogen atoms and an average molecular weight of less than 400 g/mol are, for example, aliphatic 40 diols of 2 to 12, preferably 2, 4 or 6, carbon atoms, such as ethanediol, 1,6-hexanediol, diethylene glycol, dipropylene glycol and in particular 1,4-butanediol. However, diesters of terephthalic acid with glycols of 2 to 4 carbon atoms, eg. bisethylene glycol terephthalate or bis-1,4-butanediol terephthalate, hydroxyalkyl ethers of hydroquinone, eg. 1,4-di-(.beta.-hydroxyethyl)-hydroquinone, (cyclo)aliphatic diamines, eg. 4,4'-diaminodicyclohexylmethane, 3,3'-dimethyl-4,4'-diaminodicyclohexylmethane, isophoronediamine, ethylenediamine, 1,2- and 1,3-propylenediamine, N-methyl-1,3-propylenediamine or N,N'-dimethylethylenediamine, and aromatic diamines, eg. 2,4- and 2,6-toluylenediamine, 3,5-diethyl-2,4- and -2,6-toluylenediamine and primary ortho-dialkyl-, trialkyl- and/or tetraalkylsubstituted 4,4'-diaminodiphenylmethanes, are also suitable. PA1 d) Suitable catalysts (d) which may be used in particular in order to accelerate reaction between the NCO groups of the diisocyanates (a) and the hydroxyl groups of the components (b) and (c) are the conventional tertiary a/nines known from the prior art, for example triethylamine, dimethylcyclohexylamine, N-methylmorpholine, N,N'-dimethylpiperazine, 2-(dimethylaminoethoxy)ethanol, diazabicyclo[2.2.2]octane and the like, and in particular organometallic compounds, such as titanium compounds, for example titanic acid esters, iron compounds, eg. iron(III) acetylacetonate, lead compounds, such as lead acetate, tin compounds, eg. tin diacetate, tin dioctoate, tin dilaurate or the dialkyltin salts of aliphatic carboxylic acids, such as dibutyltin diacetate, dibutyltin dilaurate or the like. The catalysts are usually used in amounts of from 0.001 to 0.1 part per 100 parts of polyhydroxy compound. PA1 e) Compounds which are less than difunctional with respect to isocyanates and/or less than difunctional isocyanates (e) may be required to avoid excessive crosslinking of the thermoplastic polyurethane, as may be caused by the use of polyisocyanates having a higher functionality. PA1 f) The conventional assistants and/or additives can of course be incorporated before and/or during and/or after the polyurethane reaction. Examples are lubricants, inhibitors, hydrolysis stabilizers, light stabilizers, heat stabilizers, stabilizers against discoloration, dyes, pigments, inorganic and/or organic fillers, reinforcing agents and plasticizers, as well as plastics which can be melted by a thermoplastic method. PA1 n--is the speed of the extruder screws in [min.sup.-1 ], PA1 D.sub.G --is the smallest internal diameter of the extruder barrel in [cm] and PA1 V--is the free reactor volume available between the extruder barrel and the screw elements and any other elements as equipment on the extruder shafts in [cm.sup.3 ],
in the presence or absence of
in a twin-screw extruder in which both screws rotate in the same direction and which has a length/diameter ratio of from 20 to 60, and discharging the resulting thermoplastic polyurethane from the extruder with shaping.
The preparation of plastics in extruders is generally known. In contrast to the processing of plastics by a purely thermoplastic method, the term reactive extrusion is used in the case of such polymer syntheses in an extruder. Accordingly, the extruder used as the chemical reactor is frequently also referred to as a reaction extruder.
In the monograph Reactive Extrusion: principles and practice by MARINO XANTHOS (Editor), published by Hanser Verlag, Munich, Vienna, New York, Barcelona (1992), important, hitherto known aspects of reaction extrusion are described. The synthesis of polyurethanes in the reaction extruder is described in Section 4.2.3a, page 87 et seq., of the stated monograph. The production of the prior art polyurethanes relevant to the invention is described in Kunststoff-Handbuch, Volume VII, Editors VIEWEG and H OCHTLEN, Carl Hanser-Verlag Munich, 1966, and in the subsequent Volume 7, Editors BECKER and BRAUN, Carl Hanser-Verlag, Munich, Vienna, 1993. Most of these substances have a more or less elastomeric character.
Various processes are known for the industrial production of polyurethanes in the reaction extruder and are described, for example, in DE-A-20 59 570, 23 02 564, 24 37 764, 24 47 368, 25 49 372, 28 42 806, 28 54 409, 29 25 944, 39 31 419, 40 17 571 and 42 02 973.
Virtually without exception, the literature recommends the twin-screw extruder in which both screws rotate in the same direction, for the synthesis of polyurethanes. Single-screw extruders and twin-screw extruders having counter-rotating screws are evidently unsatisfactory owing to the poor mixing effect during passage through the extruder, and extruders having more than two screws are too expensive.
Shortly after the raw materials required for the synthesis have been fed into the extruder, the reaction mixture has a very low viscosity and the mixing effect of the extruder screws in the conventional processes is poor in this phase. Inhomogeneities which vary in extent are formed in the polyurethane melt.
The avoidance of inhomogeneities by means of special designs of the screw geometry is described in a number of German Laid-Open Applications, for example in DE-A-23 02 564, 24 23 764, 25 49 372 and 28 42 806. The common feature of these processes is a screw configuration with a large number of kneading elements which have a highly shearing action and in particular introduce a considerable amount of energy into the reaction melt in a critical reaction phase in which the reacting mixture has viscosities of from 10 to 100 Pa.s. In the stated monograph by M. XANTHOS (Editor), Bruce BROWN likewise presents this configuration, the large amount of kneading units in 3 zones being regarded as the key to the success of this process. Virtually all subsequent publications on the synthesis of polyurethane elastomers start from this screw configuration or from a similar configuration of the extruder screws with a relatively large number of highly shearing kneading elements which have an intensive action on the reaction mixture at relatively high melt viscosities above 10 Pa.s.
Although gel-like nodules can be more or less prevented in the stated manner using this screw configuration, insoluble deposits which comprise hard segments, are insoluble even in a mixture of dimethylformamide with 1 percent of di-n-butylamine, become detached gradually and cause considerable contamination, particularly in the case of harder formulations, form particularly on the kneading elements, especially after the extruder has been running for a relatively long time, ie. for several hours or days. When a melt filter is used, these particles, which are often dark but may also be lighter, are not always completely retained since, owing to their resilience and deformability, they can even pass through filters having a small mesh size and can remain in the polyurethane and are at least visually unattractive.
Only a short time after the beginning of the synthesis, ie. after the extruder has been running for about 2 hours, the amount of inhomogeneities which is retainable on the melt filter having a mesh size of 42 .mu.m is more than 10 grams per ton of melt. After a longer machine running time of from about 15 to 20 hours, this value often increases considerably.
Even if these impurities can be retained with the aid of a melt filter, increasing coating of the filter gives rise to pressure rises in the melt, which are very disadvantageous with regard to a uniform course of the polyurethane synthesis and adversely affect the uniformity of the polyurethane quality. The required frequent changing of the filters results not only in product losses but once again in sudden pressure changes with adverse consequences for the polyurethane quality. In the case of contents of more than 1 ppm of inhomogeneities, it is by no means advisable to omit the melt filter since this amount of impurities in the polyurethane is generally no longer tolerated. The attempts to reduce the formation of inhomogeneities to the stated order of magnitude of less than 1 g per ton of melt with the aid of very thorough premixing of the reactants, as described, for example, in DE-A-42 02 973, is also unsuccessful. It is known that separation occurs even from an ideally mixed reaction batch of a polyurethane in the course of the reaction, and hard segments formed from polyisocyanate and chain extenders separate out.
This behavior is described, for example, by G. ZEITLER in the documents of the World Polyurethane Congress of 1987, Verlag Technomic, Lancaster, Basle 1987, page 148 et seq., and also by KN ONER et al. in Plaste und Kautschuk, 33, page 127 et seq. In the reaction according to the conventional processes for polyurethane synthesis in a twin-screw extruder, in particular, urea groups formed from primary or secondary amines, even from traces of about 0.01% of water in the polyhydroxy compounds with isocyanate, can form inhomogeneities which no longer melt and which far exceed the value of 1 ppm in the synthesis in the reaction extruder.
In WO 91/00304, only a single mixing zone having a high shearing effect in the viscosity range above 100 Pa.s with strongly shearing kneading elements is regarded as essential. Apart from the fact that polyurethanes are prepared without, or virtually without, oligomeric diols or diamines in this manner, it is true both for this special case and for the polyurethanes of elastomeric character, ie. containing a relatively large amount of oligomeric diols, that the proposed screw geometry and the associated process conditions permit the production only of polyurethane melts having more than 10 g of inhomogeneities per ton.
Maintaining a constant viscosity according to DE-A-20 59 570 requires a reduction in the temperature in upstream zones of the extruder with polyisocyanate conversions of less than or slightly more than 50%.
It is known, for example from the stated publication by KNONER et al., that the rate of formation of hard segments, even from ideally mixed reaction batches, passes through a maximum with increasing temperature in this conversion range, which maximum becomes more and more pronounced with increasing isocyanate content at higher temperatures. According to DE-A-20 59 570, a large proportion of conventional formulations for polyurethanes suffer from the risk that, as the temperature in the upstream zones of the extruder decreases, the rate of separation of hard segments increases in a catastrophic manner and an extremely large amount of inhomogeneities are formed.